1. Field of the Invention
The present invention relates to a rectifier circuit having a function as a limiter. Further, the present invention relates to a semiconductor device which can communicate wirelessly with a voltage rectified by the rectifier circuit, and a driving method thereof.
2. Description of the Related Art
Technology by which a signal is transmitted and received without contact between media (tags) each incorporating an integrated circuit and an antenna (RFID: Radio Frequency Identification) has been put to practical use in various fields and has been anticipated to further expand the market as a new mode of information communication. As the shape of a tag used for an RFID, a card shape or a chip shape which is smaller than such a card shape are adopted in many cases; however, various shapes can be adopted in accordance with an application.
As for RFID, communication between a tag and a reader can be realized with a radio wave. Specifically, a radio wave generated from a reader is converted into an electric signal by an antenna in a tag, and an integrated circuit in the tag is operated in accordance with the electric signal. Then, the radio wave modulated in accordance with the electric signal output from the integrated circuit is output from the antenna, and thus a signal can be transmitted to the reader without contact.
It is to be noted that tags can be broadly categorized into two types: active tags and passive tags. An active tag incorporates a primary battery and does not generate electric energy in the tag.
Meanwhile, a passive tag can generate electric energy in the tag with the use of a radio wave from a reader. Specifically, after a radio wave received from the reader is converted into an AC voltage by an antenna, the AC voltage is rectified by a rectifier circuit and then supplied to each circuit in the tag. Therefore, since energy of a radio wave which can be received by the antenna is higher, higher electric energy can be generated. However, a prescribed intensity of a radio wave to be generated from a reader is set accordingly and thus, electric energy to be generated in a tag can generally be in a predetermined range.
However, when a radio wave from a reader includes a noise or unnecessary radiation is emitted from an electronic device other than the reader, the tag might be exposed to an intense radio wave which exceeds a predetermined level. In that case, such an excessively high AC voltage that deviates from a predetermined range is generated in an antenna. Accordingly, the value of a current to be supplied to a semiconductor element in an integrated circuit might be drastically raised, which results in destroying or deteriorating the integrated circuit due to dielectric breakdown.
In particular, in a case where communication is performed with the use of a radio wave with high frequency in order to extend a communication distance, a semiconductor element included in an integrated circuit tends to be miniaturized so that the integrated circuit can be operated more rapidly. However, when the semiconductor element is miniaturized, a withstand voltage becomes lower, and further, a tag is easily damaged due to an excess current.
In addition, when intensity of a radio wave in a case where a communication distance between a tag and a reader is short and that in a case where the communication distance between the tag and the reader is long are the same, electric energy generated in a tag increases as a communication distance between a tag and a reader shortens. Therefore, when the communication distance is short, excess electric energy might be generated.
It is very effective to provide a limiter having a function of discharging excess electric energy in an integrated circuit in order to improve reliability of a tag. A limiter has a function of controlling a voltage to be output to be lower than or equal to a set voltage (limit voltage) regardless of an input voltage. With such a limiter, the above-described deterioration or destruction of a semiconductor element due to an excess current is prevented.
In a case of a tag, a circuit of a semiconductor element which is most easily deteriorated or destroyed is a circuit to which an AC voltage is directly input from an antenna. A rectifier circuit is one of such a circuit, and a rectifier circuit provided with a diode functioning as a limiter on an input side is described in Patent Document 1: Japanese Published Patent Application No. 2002-176141 (FIG. 1 in the page 6).
However, in a case where a limiter is provided between an antenna and a rectifier circuit, there is a problem in that the rectifier circuit is short-circuited with a ground (GND) side due to parasitic capacitance or parasitic inductance in the limiter and power is consumed by the limiter even when excess current is not generated.
FIG. 19A shows a general structure of an antenna 1901, a limiter 1902, and a rectifier circuit 1903. The limiter 1902 and the rectifier circuit 1903 correspond to part of an integrated circuit, and the limiter 1902 is connected to a terminal A1 and a terminal A2 of the antenna 1901. In addition, the rectifier circuit 1903 is connected to a next stage of the limiter 1902.
FIG. 19B shows an equivalent circuit diagram of the antenna 1901, the limiter 1902, and the rectifier circuit 1903 which are shown in FIG. 19A. Note that FIG. 19B shows a circuit diagram in a state where the terminal A2 is connected to GND. The antenna 1901 includes an inductor 1910 and a resonant capacitor 1911 which are connected in parallel. The limiter 1902 includes a switch 1912 which controls connection between the terminal A1 and the terminal A2. In addition, the rectifier circuit 1903 is connected to the terminal A1 and the terminal A2 in the next stage of the limiter 1902.
When a voltage lower than a prescribed voltage is applied between the terminal A1 and the terminal A2, the switch 1912 is opened and the voltage between the terminal A1 and the terminal A2 is applied to the rectifier circuit 1903 as is. Conversely, when a voltage with a larger amplitude than a predetermined voltage is applied between the terminal A1 and the terminal A2, the switch 1912 is short-circuited and thus an excess current flows to the terminal A2 (GND) side, so that a voltage to be applied to the rectifier circuit 1903 can be suppressed.
Incidentally, since the switch 1912 is generally formed using a semiconductor element such as a transistor or a diode, it includes parasitic capacitance or parasitic inductance. Therefore, when an AC voltage with high frequency is applied between the terminal A1 and the terminal A2, the AC voltage is applied to the terminal A2 side as well due to parasitic capacitance or parasitic inductance and thus power is lost even if the switch 1912 is open. The current situation is that an excess loss of power is desirably minimized because electric energy which can be generated in a tag is limited.